Special Topics

Answers to questions that have been posed over the years.

The October 1987 storm - a 'hurricane', or not?

Well, it all depends who you talk to! Meteorologists make a clear distinction between hurricanes, a regional name given to tropical cyclones occurring in the tropical Atlantic and east Pacific basins; and intense mid-latitude storms. Reference to the following will show why:-

Hurricanes, or tropical cyclones, form in an environment of little or no vertical wind shear. Vertical shear (which doesn't have to be throughout the troposphere, but can also be over a very shallow layer) destroys the convection around the centre of the tropical cyclone (the 'eye'). For a tropical cyclone to continue to develop, there must be inflow of warm air at lower levels, and upper level outflow: the convection provides the 'pathway' for the necessary rising air. They form over very warm waters - sea surface temperatures (SST) values above about 27 degC, equator-ward of the sub-tropical anticyclone belt, with the capture/inflow of water vapour and sensible heat from the tropical oceans being essential to the physics of the storm. The storm is warm core, especially in the lower troposphere, with little overall (synoptic scale) 500-1000 hPa thickness gradient, and therefore hurricanes have no 'Norwegian model' fronts associated with them. Low pressure is primarily due to the contrast between the warm core of the storm, and the unperturbed tropical environment, with some contribution from compressional warming of descending air within the 'eye' of the storm, and the speed of movement of the storm is generally less than 15 knots.

(NB: the name 'tropical' should not be taken to mean that these features only form between the Tropics of Cancer and Capricorn; they can develop well poleward of the 'tropics' and indeed persist with tropical characteristics [up to 'Hurricane' category], well into the normally accepted 'mid-latitudes': it is the storm structure - as defined above - that is important, not the exact location of the disturbance.)

Mid-latitude severe storms form in a strongly sheared environment, such as is found at high levels around the parent Polar Front jet core - jetstreams play no (direct) part in the formation of a hurricane. The formation of a mid-latitude storm is triggered by a short wave eastward moving disturbance embedded in the upper flow, with consequent distortion of the pre-existing baroclinic (i.e. frontal) zone. There is a strong 500-1000 hPa gradient involved. There is appreciable disturbance of the tropopause in the vicinity of the storm, particularly to the rear in the 'dry slot', and this is thought to be important in that it indicates the intrusion of dry stratospheric air - a key ingredient in the 'explosive cyclogenisis' aspect of these storms. (At the present time, it is not known for certain whether stratospheric air is involved with tropical cyclones: studies are underway to investigate this). The low (or lowering) pressure at the surface is due to an excess of divergence of mass aloft over convergence below, coupled to strong warm advection. The speed of movement of such storms are often in excess of 30 knots.

So far, so good - meteorologists are not going to get the two phenomena mixed up, but when looking at the October 1987 storm that hit the southeast of England, these clear scientific differences must be balanced against the reality of the event. For example, some very warm/moist air was entrained in the storm, possibly the remnants of a former tropical cyclone. Although the 10 minute mean winds in most cases failed to reach the threshold of 64 knots for a hurricane, two reports within the circulation over/adjacent to the English Channel did exceed this threshold, and although not analysed, 1 minute means, which the Miami NHC uses to classify hurricanes, almost certainly would have reached or exceeded this level, particularly when set against observed gusts of 70-90 knots or more, which are easily attained in mature tropical cyclones. There was widespread damage and disruption, with millions of trees damaged or felled, several people dead, ferries stranded on windward shores and given these facts, it easily matched the OED definition of a hurricane.

Prior to dawn on the 16th October, 1987, the image most members of the general public had of the damage wrought by hurricanes came from television pictures from the US or the Far East. The folk of the south east of England then are surely to be forgiven if venturing out and finding the car under a substantial tree, or whole communities cut off from electrical power, they refer to this event as "... the 'hurricane' of 1987".

(help with information relating to tropical cyclones was supplied by: Sim Aberson, a meteorologist with NOAA's Hurricane Research Division in Miami, Florida.) For more detail, visit the Tropical Cyclone FAQ at: http://www.aoml.noaa.gov/hrd/tcfaq/tcfaqHED.html.

[ For more on the use/abuse of the phrase "Hurricane Force 12" see:- HERE ]

What does the terminology in the Shipping Forecast mean?

The Shipping Forecast, which is provided by the Met Office (under a contract with the UK Maritime and Coastguard Agency), and broadcast four times daily on BBC Radio 4, is highly structured to maximise the use of the available time. The basic order of the forecast is:

- GALE WARNINGS IN FORCE

- GENERAL SITUATION

- AREA FORECASTS: WIND DIRECTION/SPEED: (SEA STATES**): WEATHER: VISIBILITY: (SHIP ICING IF APPROPRIATE)

- COASTAL WEATHER REPORTS AROUND BRITISH/IRISH COASTS (*)


(*) From April 6th, 1998, certain bulletins no longer carry coastal weather reports.

(**) from 2006, some versions of the bulletin under header FPUK71 EGRR have included sea states.

Most of the forecast is self-explanatory, but in the synoptic preamble, and in the weather reports which follows, some terms are used which may not be familiar.

Movement of pressure centres: (in forecast preamble/general situation)

Slowly   up to 15 knots   (approx: up to 8 m/s or 28 km/hr)
Steadily   15 - 25 knots   (approx: 8 - 13 m/s or 28 - 46 km/hr)
Rather quickly   25 - 35 knots   (approx: 13 - 18 m/s or 46 - 65 km/hr)
Rapidly   35 - 45 knots   (approx: 18 - 23 m/s or 65 - 83 km/hr)
Very rapidly   over 45 knots    (approx: over 23 m/s or 83 km/hr)

 

Pressure changes:(in coastal station reports/3 hours is a 'standard' time period used in synoptic meteorology in mid/high latitudes.)

Steady   Change less than 0.1 mbar in past 3 hours
Rising/Falling slowly   Change 0.1 to 1.5 mbar in past 3 hours
Rising/Falling   Change 1.6 to 3.5 mbar in past 3 hours
Rising/Falling quickly   Change 3.6 to 6.0 mbar in past 3 hours
Rising/Falling very rapidly   Change more than 6.0 mbar in past 3 hours

 

Veering/Backing of wind: When a wind direction changes such that it moves with the clock, e.g. from east to south through south-east, that is a veering wind; A wind therefore that changes against the normal clock motion is a backing wind.

...and for the visibility categories the following apply:

FOG   < 1 km   < 1100 yds
POOR   1 to 3.9 km   1100 yds to 2 nautical miles
MODERATE   4 to 9 km   2 to 5 nautical miles
GOOD   >=10 km   > 5 nautical miles

 

Since 2006, sea states (strictly wind-wave forecasts) have been included in some versions of the bulletin: they employ a crude relationship between the forecast wind and expected wind wave heights (mean of a well-formed wave train) IN OPEN WATER. The categories are as under:-

Description 
  Height in metres
Calm   0.1 or less
Smooth   >0.1 to 0.5
Slight   >0.5 to 1.25
Moderate   >1.25 to 2.5
Rough   >2.5 to 4.0
Very rough   >4.0 to 6.0
High   >6.0 to 9.0
Very high   >9.0 to 14.0
Phenomenal   >14.0

 

Click here for the LATEST FORECAST.

... and for more on terminology (including what a 'perhaps ... later' gale means), see Frank Singleton's site from here.

Is there anywhere I can go to look up past weather information and refer to specialist meteorological literature?

As part of the Public Met. Service, the Met Office maintains the National Meteorological Library and Archive (NML&A), which are open to all, particularly those with an interest in meteorology, both amateur and professional. The main facilities are located at Exeter, Devon (see below for Scotland and Northern Ireland). [Historical note: prior to autumn 2003 (Library) and autumn 2004 (Archives), these facilities were in Bracknell, Berkshire.)

use this url to obtain more details:-

http://www.metoffice.com/corporate/library/index.html

(NOTE: there is now an on-line search facility available via this url)


The Library houses weather summaries extending back well into the 19th century, and has an excellent collection of literature, covering most of the earth sciences. It also holds most of the specialist scientific journals on the subject - several from volume 1 number 1, e.g. Weather, Meteorological Magazine, Weatherwise, Journal of Meteorology etc. The Archive holds weather charts from 1867 and observation registers for many sites at home and abroad. (For Met.code buffs, the Library also holds copies of the WMO international coding manuals.)

It would be advisable, before making a lengthy journey, to contact the Library Information desk to discuss your requirement and confirm opening times etc. See the web site for contact details.

(Email:

metlib@metoffice.gov.uk for the Library

metarc@metoffice.gov.uk for the Archive)

Archives for Scotland & Northern Ireland are held in Edinburgh and Belfast respectively.

When was the concept of an "air mass" proposed?

Not long after the electric telegraph made simultaneous (i.e. 'synoptic') observations possible in near 'real time', it was realised that in regions of 'disturbed' weather, (i.e. close to what we now call a depression), two different 'streams' of air could often be found converging into the disturbed zone - each having markedly different properties.

In the British Isles, Robert FitzRoy, the first director of the Meteorological Office is usually credited with highlighting this fact in 1863, though other workers, particularly in France, Germany, Holland and the United States were thinking along the same lines at the same time. Upon the death of FitzRoy, the concept tended to falter, until later workers took up the theme and elaborated upon it: Abercromby in 1887, Napier Shaw and Lempfert in 1911 and of course by the 'Bergen school': V and J Bjerknes and H. Solberg and others during and just after the Great War.

These latter workers proposed the now familiar 'Norwegian model' of the life-cycle of a mid-latitude depression, whereby a minor wave develops along the boundary between two well defined air masses, amplifies (develops) and is carried forward in the general flow. The poleward air mass has an east-to-west component of air motion at low levels, is relatively cold (ex. Polar), and therefore dense, and has a relatively lower humidity value (lower dewpoint) than the 'opposing' air mass. This latter has a generally west-to-east component of motion (at all levels in the troposphere), is warmer (ex. sub-Tropical) and therefore lighter, and has a higher humidity/dew point value. The colder air mass was designated Polar Maritime, and the warmer air mass Tropical Maritime. The boundary between the two air masses came to be known as the Polar Front (see also here and here for other FAQs in this area).

So, how is an 'air mass' defined?

An air mass is classically defined as a large body of air (many hundreds to a few thousands of km in extent),having quasi-uniform horizontal temperature and humidity characteristics. Indeed, once upper-air soundings became available on a regular basis, it could be seen that this uniformity extended vertically, such that each air mass has a distinct vertical profile of temperature and humidity.

To attain these uniform (or nearly so - nothing is that clean-cut in meteorology!) signatures, a large body of air has to remain over one area for a considerable time - measured in weeks rather than days. This requires a pressure pattern which allows stagnation of the air - and this usually means a slow-moving anticyclone such as is found in the great sub-tropical high pressure belts, the polar high pressure regions or the Asiatic (or other great continental) winter anticyclones. These are said to be the 'source' regions of an air mass. Once an air mass leaves its source region, it is modified, depending largely upon the type and temperature of the underlying surface over which it moves.

For example, air that moves polewards from the sub-tropical high pressure belts encircling the earth will be cooled from below as it passes over progressively colder seas, and this will in turn affect the relative humidity (increasing it leading to formation of cloud/precipitation), and although these processes may slightly lower the absolute humidity, it will still have a higher humidity value than air coming from polar latitudes, which will be warmed from below and will become increasingly buoyant as heat is input to the lower layers.

Air Masses can be classified as 'polar' (having originated in cold/high latitude regions), or 'tropical' (having come out of the stagnant regions around the sub-tropical high. They are further sub-classified as either 'maritime': having passed over a sea surface, or 'continental' having moved over a land mass. This then gives rise to the four principal types of interest to us in north-west Europe:- tropical maritime (Tm or mT), polar maritime (Pm or mP), tropical continental (Tc or cT) and polar continental (Pc or cP). There are of course many modifications , and a full treatment of air masses is outside the scope of this FAQ. See the list of recommended reading.

How will 'global warming' affect rainfall patterns over north-western regions of Europe?

(thanks to Keith Dancey for this answer....which is his reply to a question in the newsgroup.....)

If more heat is pumped into the system (system=earth) then more water vapour will be put into the atmosphere. How that would affect our (local) weather depends upon how it would affect the world's climate, and how the world's climate affects our (local) weather. Precise answers to these questions are not known. There are climate models which can be run, and they are improving, but they are not 100% accurate, and they may never be! Such models require knowledge of the atmosphere and oceans that are beyond us at the moment, and computing power that can represent all the processes that are going on all the time. A rather tall order. You might be interested to learn that the Gulf Stream (a natural phenomenon that defines, to a large extent, the UK's mild climate for it's latitude) might even become disrupted under certain conditions in some ocean models. So whether global warming is happening, and how far it might go, is really very important, even to us.

Global warming, per se, can be tested by measuring the average temperature of the surface of the sea, and keeping records for a long time. We have historical records, of varying accuracy and varying coverage. We now have instruments orbiting the globe that can measure the sea-surface temperature to breathtaking accuracy. The data indicates warming. The period is rather short. But we don't know (for certain) that this is because of us (human economic activity) or some natural phenomenon that we have yet to discover. Most scientists working in the field believe the former. Increased rainfall (and other local climate change) for the UK and Europe can indeed be an outcome of global warming. When a possibly chaotic system such as the world's climate is perturbed, it might be impossible to predict the outcome, other than there is going to be change. Global warming does not necessarily mean "drier". It certainly does not mean "drier everywhere". And it also does not mean "warmer everywhere".

Is there a system of classifying synoptic weather types over the British Isles?

(NB: 'synoptic' in meteorology is used in the sense that the weather is analysed over a wide area at approximately the same time.)

A. In the early 1950's, Hubert Lamb expanded upon a classification system originally proposed in an article in 'Weather', into the now widely used Lamb's circulation types. The late Professor Lamb(*) was responsible in the UK for much work involved with deciphering the climatological changes that have undoubtedly occurred, and will continue to occur. Indeed, in the early days, the work was rather unfashionable, but is now required study given current concerns. Professor Lamb consolidated his distinguished career by taking a professorship, and the post of first director, at the University of East Anglia's Climatic Research Unit. (see the section on climate change). The system is based on the analysis of the direction of the overall isobaric pattern (not the individual wind direction at any one place) over the region 50-60N, 10W-02E. Once one of the 8 compass point directions, from which the wind blows is allocated, the curvature of the flow is considered, and the directional letters are prefixed by either: A, anticyclonic or C, cyclonic or it is left unclassified (neutral or irregular), when a qualifying letter is not used. Three other categories are recognised: A=anticyclonic (i.e. a notable high pressure itself over the region), C=cyclonic (i.e. a notable low pressure over the region), or U=unclassifiable. This gives rise to 27 classes.

Visit the UEA site to find out more, and to view the catalogue maintained by them.

http://www.cru.uea.ac.uk/~mikeh/datasets/uk/

(*) Professor Lamb died on Friday, 27th June, 1997

How can I obtain details about periodicals/magazines that are published in the British Isles which deal with meteorology?

The British Isles are well served by English language magazines, periodicals etc., that cover a wide range of interest in the subject, from the keen amateur to the 'cutting-edge' academic end of the spectrum. More details are set out in the section on reading material. Also, several sites listed elsewhere have good links/information on such matters: for example:

The Royal Meteorological Society: http://www.royal-met-soc.org.uk/

and Roger Brugge's site: http://www.met.reading.ac.uk/~brugge/

Would anyone be interested in my weather observations?

Indeed YES! For a start, the uk.sci.weather newsgroup itself is always a good place to post if you've seen something that might be of interest, particularly in the 'unusual' or 'severe' category. The best way to approach this is to 'lurk' for a short while to get an idea of what interests us, then dive in when you feel happy. If you are interested in the 'weather', this is the place for you. To try and help, there is a complete section in this FAQ which deals exclusively with weather observing ... not intended to be exhaustive, but might give you some ideas, tips etc.

To aid those who do not want to plough through observational data, we have a system of 'indicators' [WR] and [OBS] used in the Subject line of any thread containing observational remarks; for more on this see here.

With notable 'convective' weather, e.g. a decent thunderstorm, whirlwinds, tornadoes, etc., then TORRO (see here) would be interested in a report. Visit their site at: http://www.torro.org.uk/

And, how about becoming a member of the Climatological Observers Link? This organisation has been in existence since 1970 and is open to anyone with an interest in meteorology. Both amateurs and professionals are registered observers. Visit: http://www.met.rdg.ac.uk/~brugge/col.html for more details.

Also, a UK Weather Diary can be viewed and added to, which can be seen at: http://www.met.rdg.ac.uk/~brugge/diary.html

  [ Although the national domain is 'uk', don't be put off reporting/joining up to the above outside the United Kingdom. The weather knows no national boundaries! ]

And now (autumn, 2000), the BBC Weather Centre is keen for you to email weather observations direct to them ... they will take reports on: emet@bbc.co.uk

 

What is the North Atlantic Oscillation?

(This note prepared with the help of: Dr. Rob Wilby, Department of Geography, University of Derby.)

In very crude terms, it is possible to visualise the mean sea level pressure patterns affecting the north-east Atlantic as varying (or 'oscillating') between two extremes: At one extreme is a minimally perturbed westerly type, with disturbances rattling swiftly across the Atlantic, hurried along by very strong mid / upper tropospheric winds. At the other extreme lies a weak, perhaps ill-defined pressure pattern, but with a strong tendency for stagnation of weather types over and downwind of the north-east Atlantic (a persistent 'blocked' type). [ use of the word 'oscillation' in the popular mind implies some sort of regularity, which in reality is not observed, at least not on short time scales. ]

One method is to use a measure of the 500 mbar strength between defined latitudes: By taking the difference between mean 500 mbar contour heights between latitude 35 and 60 North, this simple method yields high numbers ( a high zonal index ), for strong westerly types, and low numbers ( a low zonal index ), for weak westerly, or blocked types. Another method would be to categorise circulation types using, for example, Lamb's Weather Classification.

However, a simple measure, using observed msl pressure differences from long-term 'normals', can be employed, and can of course be extended back to times well before upper air information became routinely available. Upwind of the British Isles / NW Europe, stations in Iceland and the Azores (and / or Gibraltar) are used, by convention, to define the North Atlantic Oscillation Index (NAOI).

The method of calculation means that lower than normal mslp over the Iceland region and / or higher than normal mslp in the Azores / Gibraltar zone gives rise to a +ve NAOI. The converse situation gives rise to -ve NAOI values. The Index is of most use during the winter, when highly positive values are associated with warmer, wetter, windier winter seasons, especially over NW Europe.

In a record since 1823, the following major 'divisions' can be identified in the winter (December, January, February) season:

  • 1830's to ~1900: +ve overall, but with fairly frequent -ve seasons.
  • 1900 to first half 1930's: strongly +ve (similar to modern times) with only 1 marked -ve season (1916/17).
  • second half 1930's to ~1970: a gentle decline overall, with a series of strongly -ve phases; includes the severe events of 1947, 1962/63 & the second most -ve winter of 1968/69.
  • from 1970: irregular, then from 1980 (@/see note below) strong rise to current generally +ve phase; few strongly -ve winters after 1980, BUT the most -ve winter in the entire series occurs in 1995/96 and it is interesting to note that the long-term 'moving average' has shown a downward turn in recent years.

( @ Note: The strongly positive NAOI since the 1980s has resulted in a higher frequency of unusually wet and mild winter conditions over most of NW Europe and Scandinavia during this period, with concomitant changes in regional runoff.)

to see data relating to the NAOI go to: http://www.tiempocyberclimate.org/portal/datanao.htm and other interesting links therefrom.

What is the Central England Temperature series?

In studies of the climate for any region, locality etc., it is important to have a homogeneous record to describe such atmospheric variables as rainfall, sunshine, temperature which eliminate as far as possible changes in site (both location and characteristics), observing practice and so on. Professor Gordon Manley (1902-1980) developed one such series which dealt with the temperature of 'central England', defined as the area stretching from the Lancashire Plain southwards across the Midlands, and constructed using stations in the Lancashire (including the modern-day Merseyside/Gtr.Manchester) area, and the east and west Midlands.

The series has been maintained since his death, although there have been changes in stations used, as some have closed/altered. Corrections also have to be applied, particularly to latter-day observations to take account of urbanisation, however the 'CET' series remains one of the longest and most widely used of its kind in the world. The record now extends, on a monthly basis, back to 1659, and on a daily record back to 1772. However, the values prior to 1721 are regarded as less reliable than later data, a fact acknowledged by Manley amongst others. Nothwithstanding this caveat, useful clues to changes in climate can be gleaned from this work. It has also been shown that the CET series is a statistically useful indicator of changes of mean temperature for a somewhat wider area than just the 'English Midlands'.

To see the monthly series maintained by the University of East Anglia go to:

http://www.cru.uea.ac.uk/~mikeh/datasets/uk/

... and for data, and more information on the data-set maintained by the Hadley Centre/Met Office, go to:

http://www.metoffice.gov.uk/research/hadleycentre/obsdata/cet.html

Remember though that values are often revised after initial issue: it is a good idea to check back periodically to capture such changes.

What impact does 'El Nino' have on the weather over Europe?

The 'El Nino' phenomenon, or more strictly the warm El Nino -Southern Oscillation (ENSO) event is coupled closely to remarkable shifts in weather patterns in the immediate Pacific basin, and adjacent areas: e.g. parts of North America. For example, it is clear that the altered distribution of warm/cold water across the equatorial Pacific is the primary reason why excessive rain can fall in places like Peru, and a general deficit of rainfall is experienced in Indonesia, parts of Australia and the Philippines. There is also a generally accepted link between a less-than-'normally' active Atlantic hurricane season and the notably warm event that characterises what has come to be called, THE El Nino.

It is becoming clear from recent studies that we can now rule out the 'No Effect' case: this leaves us with two options -

(a) There IS an effect, but it is on a scale that is dwarfed by regional variations closer to home, e.g. long-term thermal inertia in SST distribution in the N. Atlantic, or continental/oceanic temperature differences across the North America - North Atlantic - Eurasian 'super-region'.
(b) There is a direct, and marked effect that leads to verifiable modification of the weather types across the NE Atlantic/European - Mediterranean region.

(a) appears to be the most likely if we take the year overall; indeed, even in studies published which set out to prove the link between warm/cold ENSO regimes, and impacts over Europe, caution is always advised relating to local/regional scale modification.

(b) is climbing higher in the 'probability' stakes, at least if the 'winter' season only is considered. There are an increasing number of studies published that show a direct link between a warm ENSO season, and, for example, altered rainfall/temperature anomalies across west/central Europe. No lesser person than J.Bjerknes postulated in 1966 that altered activity in the equatorial Pacific appeared to significantly alter the strength/orientation of the PFJ over and downwind of the NE Pacific, which in turn must have at least some effect on the long-wave structure downstream. This appears to have been accepted in later studies & developed further using datasets going back over two centuries or more.

However, this topic will be kept under review, as will this Q/A, and our ideas may change ... for the moment though, for more on El Nino/ENSO etc., see the following sites:

[WMO home page]
http://www.wmo.ch/

[El Nino theme page sponsored by NOAA/TOGA-TAO]
http://www.pmel.noaa.gov/toga-tao/el-nino/nino-home.html

... and of course, a search of the WWW will throw up many active sites dealing with El Nino.

In addition, on this site there is set out in summary format some of the arguments/references that subscribers to uk.sci.weather asked for. See it here.

What are 'sferics', and how are they obtained?

When a lightning discharge occurs, radio waves are emitted over a broad spectrum of frequencies. For the vast majority of people, such 'atmospherics' (or 'sferics') are simply a nuisance, leading to the familiar 'crackle' that can be detected on a home radio set, particularly in the 'AM' medium or long wavebands.(@see note 1 below)

However, in the 1920's and 1930's, Robert Watson-Watt, a British scientist (and sometime employee of the UK Meteorological Office), developed a method of displaying the discharge information on a crude cathode ray tube, and by taking simultaneous observations on the same flash, the source could be located with reasonable accuracy. (@see note 2 below) This triangulation method continued in use in the United Kingdom until 1988.

The system now employed by the UK Met Office is the Arrival Time Difference (ATD) system. The origin of the lightning flash is computed from the time difference of an atmospheric arriving at several widely-spaced 'listening' stations - located across the UK, the Mediterranean & northern Europe. Each lightning stroke has an individual signal, or wave-form (in the Very Low Frequency part of the electro-magnetic spectrum), and by using accurate (atomic) clocks, and synchronisation between the detector stations, and the control station (Exeter, Devon (UK), originally at Beaufort Park, near Bracknell), an accuracy of at least 5 km (often down to 2 km) can be achieved, although in GTS SFUK bulletins (known within the Met Office as SFLOCs - SFeric+LOCation), the accuracy is limited by the code form to 0.5 deg lat/long.

One of the listening locations is automatically designated as a 'reference' (by the control station) for each discharge, and reports the time at which it detects a 'sferic' event to the control. The control station then 'asks' the remainder of the outstations for detailed wave forms of sferic events close to this time and calculates the time differences and so computes a set of possible locations. Provided more than three outstations are active (out of the 7 available) in acquiring that event, a unique location can be determined for that particular return.

The system is highly effective, though on a few occasions, 'spurious' returns can be seen (usually easily eliminated by reference to IR imagery); also when very active clusters are detected, isolated events elsewhere may not be adequately resolved - though these are now rare events. In addition, be wary of occasions when lightning returns 'appear' to all stop at once; this is usually due to a failure of the central computer, which needs re-setting before the system will function properly.

The system is fully automatic, and theoretically can detect lightning over a large portion of the globe. The system is in a state of continuous development - a major upgrade having been put in place in Autumn, 2001.


For more on the system see the Met Office Education page at this link:- http://www.metoffice.gov.uk/education/secondary/students/lightning.html


(@1:This means that an ordinary home radio set can be used as a crude lightning detector, by tuning to a portion of the waveband -- try the LW section -- that is not used by a broadcast station. During lightning activity, irregular crackles will be heard, and with a little experience it will soon be possible to pick out 'close' from 'distant' discharges by this method - a good reason to hang on to your old portable radios after the 'digital revolution'! )

(@2:This use of triangulation of signals was later adapted in his method of aircraft detection used during the early part of the second World War.)

How do I set my barometer?

Unless you are situated at some considerable altitude ... say above about 3000 ft (about 1000 m), then it is best for *home* use to have your barometer indicate the pressure at mean sea level. You can then relate your reading to those in newspapers, television charts etc. However, you should not expect a high degree of accuracy when using many barometers bought for 'decorative' use, and if you intend making weather reports for the synoptic network using a precision aneroid barometer (or similar), then the appropriate professional authority that collects and checks weather data should be consulted - the procedure is very different and involves careful periodic checking against a reference barometer and the use of correction tables/algorithms. (See also the Observer's Handbook)

For most people though the following will suffice:.... Choose a day when the atmospheric pressure is not changing greatly...in association with a slow-moving anticyclone is best (but see also below re: checking over a range). Log onto a site that gives out hourly METAR reports (see UK Weather Information for soem good sites), and pick a station/airfield nearest to your location. If there is no such location, then you may have to plot out several reports for the same time...draw a few simple isobars...then interpolate to find a value. With most home barometers, the nearest whole millibar is about the most you can expect in accuracy. Adjust the barometer by (usually) turning a recessed screw to the rear of the unit until the reading is correct. Keep tapping (gently!) the barometer to overcome friction within the mechanical linkage. Replace the barometer in a shaded/indoor location free from the possibility of accidental damage etc.

You should try and maintain, for say a month, a check against an adjacent site over a wide range of pressure values. By logging your values against those of this nearby site, you will be able to see if there is a systematic or random error in your reading. The former can be allowed for by slight re-adjustment or 'on-the-day' correction; the latter means you have a faulty unit, or its sited poorly -- in direct sunshine for example.

Incidentally, please take NO NOTICE of the absurd descriptive terms often placed around the dial of a barometer. When these originated is not known for sure, but it is known that Robert Hooke, the inventor of the 'wheel' barometer used such terms from about 1670: 'Change' was set at 29.5 inches; then 'Rain', 'Much Rain' and 'Stormy' at each half-inch on the lower side, and 'Fair', 'Set Fair' and 'Very Dry' on the high side. The regular spacing gives the clue to the lack of scientific credibility of such a scheme, and in my opinion they have no practical value.

Why does some rainfall leave a coloured dust on my car?

Through the action of widespread and vigorous duststorms over places such as the Sahara, huge quantities of very fine desert sand can be carried to high enough levels (around 12000 ft/4000 m), where it can be dispersed for considerable distances downwind of the source. On many occasions, such dust is so diffused vertically and horizontally that there is little or no effect observed at ground level.

However, sometimes the dust remains in sufficiently high concentrations, and can become involved with a medium level weather system, which results in the dust being transported towards such places as France, Britain and Ireland. If rain falls, the dust falls as well. This is primarily due to washing out of the dust by large raindrops. This leaves a dusty residue on car windscreens, rain gauges etc., with the most common colour being similar to old mortar: i.e. light beige, but deeper brown, orange and red hues have been observed. The effect is usually noted after light, showery rainfall, often involving medium level instability - heavier rainfall tends to wash the evidence away. A warm, southerly (Tropical continental) low-level airstream, together with a strong southerly middle level flow (circa 700 mbar), originating from the North African area are the conditions required for such events in the northwest of Europe.

Such reports are always of interest...some guidelines are contained in this section (Notes on observing.)

What is the history behind lettering Bracknell fronts/centres?

(with effect from 1st April, 2000, the scheme whereby letters were assigned to fronts & centres on Bracknell analysis and short-range prognosis charts ceased. The 'tracking' of low pressure centres was also stopped. This was a direct consequence of withdrawal of one of the support rosters within the NMC. "NMC Bracknell" itself 'ceased to be' in 2003, when the Met Office HQ relocated to Exeter.

To avoid re-numbering the FAQ series though, I shall maintain this abbreviated entry for the time being, as there is some historical interest.)

[ the text below, and that for 2B.17, was kindly supplied by Martin Stubbs. ]


As far as is known the practice in the UK Met Office was always to allocate letters to the features on the charts (up to April, 2000) although it is thought that this may have been a numbering system which was started in about 1944 (Lettering of pressure centres, particularly areas of low pressure, may in fact date back to the latter part of the 19th century). In fact the WMO International Analysis Code (still in existence) actually caters for the identification of fronts or systems using a number defined by the code 'NN'. The actual code form is 99NNSS where NN is defined in the WMO Manual on Codes as the identity number of the system or front. Thus analyses prepared at the Central Forecasting Office in Dunstable in the 1940s and early 1950s for example, may well have had identification letters, but when coded a depression with the letter 'A' would be coded as 990100, and possibly referred to on the outstations as Depression '1'. The UK actually coded its analyses and forecasts in three different ways after the Second World War. The coded analysis/prebaratic that went out on the international circuits carried the identifier group 99NNSS (for example, a depression labelled 'A' would have been coded 990100 81297 59346 . . . etc.), the analyses that went out internally within the Met Office converted this to plain language (for example, LA 81297 59346 . . . etc.) and for the marine bulletin to the ships on the North Atlantic the identifier was dropped altogether (81297 59346 . . . etc.).

Can I have a guided tour of Met Office 'FAX' charts?

Prior to May 2003, there was a legend, 'ASXX' or 'FSXX', which identified the product as either an analysis (A), or a forecast (F) relating to the surface (S) level - strictly mean sea level - for an undefined region (XX). The words 'Analysis' or 'Forecast' are now used where required, and/or "xx hr", where xx=the number of hours from analysis time of the forecast chart, so "84hr" is the expected situation 84 hours from the analysis time.

[ The use of ASXX and FSXX dates from the time when the bulletins were coded using the International Analysis Code when ASXX/FSXX were the headers in much the same way as SMUK is the header for a bulletin of synoptic reports made at a main hour from the UK. ]

In addition, the letters EGRR used to appear on UKMO charts, which are the ICAO identifying letters which are assigned to the Exeter Operations Centre [EXO] (before late August 2003 to the Bracknell Telecommunications Centre) - these again have been dropped from 2003.

Other abbreviations include MSLP (mean sea level pressure) and UTC - universal co-ordinated time (an acronym chosen to satisfy both the English and French speaking communities since it does not have a direct equivalent in either language - in French UTC is read as temps universel coordonne). UTC is based on an atomic standard, but for all practical purposes is equivalent to GMT.

For international transfer of pictorial information the bulletins/files containing that information carry headings such as PPVA89, PPVI89 and so on. The letter P (or Q) indicates pictorial information, the second letter P indicates the information refers to pressure, the V defines the area for which the information is provided and the fourth letter indicates whether an analysis (A), or a forecast where E,G,I,J,K,M and O are for 24/36/48/60/72/96/120-hour forecasts respectively). The figure 89 refers to any parameter at sea level.

In the top-lefthand corner, there is a geostrophic scale - used to find the (theoretical) friction-less wind speed from the spacing of the isobars;(use with caution - see "How do I use a geostrophic wind scale?".

There is no longer a distance scale on the charts, but a tip to find distances ... remember that one degree of LATITUDE is equivalent to 60 nautical miles (n miles), (thus 1.5 degrees is equivalent to 90 n miles and so on) Therefore, if you want to measure off how far a depression has travelled over the period of six hours between analyses, step off the distance with a ruler or pair of dividers, then lay this distance along a line of LONGITUDE in the same area of the chart, and count the number of degrees latitude that this represents. For example, if the distance measured off is five degrees of latitude then this is equivalent to 300 n miles (i.e. 5 times 60, which is 300 n miles in that 6 hours; The overall speed of movement of the feature is even simpler to define for that 6 hours: one has only to remember that 1 degree latitude in 6 hours (i.e. 60 n miles/6hr)=10 knots; therefore if a feature 'steps-off' 3 degrees of latitude in that 6 hours, it must be moving at 30 knots.

These simple calculations can also be used to forecast the expected movement of fronts using the simple methods described in text books (for example, active cold fronts can be advected (moved) at a speed of four-fifths the measured geostrophic wind measured just ahead of the front, the vector being in the direction of the warm sector isobars. A factor of two-thirds can be applied to the measured geostrophic wind to give the expected movement of the warm front.

Isobars are drawn every 4 millibars on charts originating in the UK, the USA and Canada but note that a 5-millibar spacing is more common on charts originating in countries in continental Europe. Isobars are labelled with values in whole millibars (or hecto-Pascals/hPa), starting at 1000 hPa.

Fronts are drawn with heavy solid lines, distinguished by solid 'triangles' for cold fronts, solid 'bobbles' for warm fronts, and a mixture of the two for an occlusion. The triangles/bobbles point in the direction that the front is heading/thought to be heading and placed on alternate sides of the line when the front is quasi-stationary. Heavy lines with no such additions indicate troughs (the word 'TROUGH' may or may not be indicated beside the line). There are occasionally variations in the graphical representation of fronts. If the front is significantly weakening (frontolysis) then a cross hatch is placed across the frontal line (itself broken) between the triangles/bobbles, and if a front is considered to be forming (frontogenesis) then the solid line is replaced by dots between the 'bobbles / spikes' [ However, in my experience, its mighty difficult to tell the difference between the two! ].

An example of many of the fronts, pressure features etc., can be seen here.

On the 'medium-range' charts (e.g. T+48, 72 etc.), there are additional long-dash lines. These are the 500-1000 hPa total thickness lines at 18 dekametre intervals.

(The medium-range charts are currently listed as Additional Products within the context of the WMO Resolution 40 (WMO Twelfth Congress 1995) and may not always be available on the Web.)

The output is now produced using on-screen analysis and field modification tools. The days of hand-drawing charts for both actual and forecast purposes (at least in the UK service) has come to an end.

What is an "Indian Summer", and why is it so-named?

(this quoted directly from the Meteorological Glossary, HMSO): " A warm, calm spell of weather occurring in the autumn, especially in October and November. The earliest record of the use of this term is at the end of the 18th century, in America, and it was introduced into the British Isles at the beginning of the nineteenth century. There is no statistical evidence to show that such a warm spell tends to recur each year. "

C.E.P. Brooks, in his 'Climate in everyday life', notes that it is the counterpart of our 'Old Wives Summer', here in Europe, and tends to follow the first severe frost and to persist for several days.

It is thought that the phrase was coined by European settlers on the Atlantic coast of North America. Paul Marriott, in his 'Red Sky at Night, Shepherd's Delight', says..." strictly an Indian Summer is a lengthy dry sunny spell from late September into November. The name probably derived from the N. American Indians who relied on a similar fine spell in late autumn for harvesting. " Philip Eden, in his 'Weatherwise' (see the 'books' section of this FAQ) also ascribes this reasoning to the term. However, I have been advised of a belief in the USA that the phrase may be a rather pejorative one coined by the early settlers, which implies that a late ( autumnal / 'Fall' ) spell of warm, sunny weather is not to be relied upon: they found the native inhabitants (in their view) were not to be trusted in like fashion.

(There is yet another theory of the origin: Merchant vessels plying the Indian Ocean would have one of the 'load-lines' marked "IS" (for Indian Summer), to show the maximum load level for ships crossing that ocean in the post-monsoon fine weather season in the latter part of the year. It has been suggested that this might be the origin of the term; I have difficulty with this. The phrase can be traced back to at least 1778, yet the common marking of vessels in this way was not standardised until 1875 (Samuel Plimsoll, MP suggested the famous 'Plimsoll Line'). It is also difficult to see why the term should come to be associated with a phenomenon in North America.)

Such spells of fine, warm dry weather may be 'reliable' in the Atlantic states of the USA; this is not so for our own climate, and Marriott (amongst others) found expectation of a period of such weather in the U.K. to be misplaced.

What is a 'Polar low'?

When arctic-origin air in winter flows southward (northward in the southern hemisphere) across (relatively) warmer seas, strong surface heating acts both to enhance the degree of instability, and trigger vigorous moist convective towers. This is sufficient alone to give rise to heavy, wintry showers/cumulonimbus clusters etc., but often marked troughing, or even a closed circulation in the isobaric flow is found; the resultant low-level convergence/positive vorticity enhancement, plus the localised concentration of the latent heat energy released, enhances development within the system, and an intense (but synoptically small) area of rain, hail, sleet or snow & squally winds can result - a polar low (or polar depression or polar meso-cyclone in some texts).

The dynamics of such systems are not fully understood, and it is only with the (recent) arrival of very high-resolution satellite imagery & sensors in a wide variety of spectral bands that the detail within such systems can be studied. Even so, for operational meteorologists, careful monitoring of all available data is required; Geostationary satellite imagery has a rather course resolution at high latitudes, and the visible channels are of little use in the winter season. Polar orbiter passes (which give much higher resolution imagery) may not be frequent enough to maintain a continuous watch on developments.

Numerical models also have difficulty with such events; they are born in data-sparse regions, and most schemes 'paramaterize' convection i.e. models don't explicitly forecast each individual convective event, but rather indicate the degree of instability expected, its areal extent etc., and thus have problems going one step further and turning an area of disorganised (model) convection into an organised self-sustaining polar low/trough, where upper troughs are not the primary forcing mechanism. The one remaining Norwegian weather ship, and a handful of research and fishing vessels may be the only clues to developments taking place in, for example, the Norwegian Sea.

Polar Lows can develop, and move (in the prevailing flow) with surprising speed, and lead to considerable dislocation of normal life in regions directly affected. Preferred locations for genesis are to the west of large, slow-moving occluded depressions - i.e. those with a pre- existing rear-flank arctic flow. It may be that the geography of the regions in question play a significant part in genesis of polar lows - Dave Wheeler, who I am grateful to for checking much of the above, suggests that vortices shed by high-arctic island groups (e.g. Svalbaard) are enhanced by the land mass of Scandinavia (Norway) to the east and Greenland and its ice shelf to the west.

I want to learn more about 'the weather' - how do I go about it?

A big question, and it all depends on the area that interests you most and whether you want to extend a 'hobby' interest, or study as part of a career choice.

Many are interested in the 'weather' as an absorbing hobby, perhaps introduced to the subject via school / college (often part of the geography syllabus), or because of a sporting / recreational activity e.g. yachting, surfing or gliding. A good number of books have been written over the years and the first port of call I suggest is to go to your local lending library and see what is on the shelves. Don't be put off because a book has been written for 'sailors' or 'pilots'. These are often very well written by professional meteorologists who have a keen interest in the sport / activity involved, and will cover the elementary facts you need to know in a clear way, without the use of complex mathematics.

At some stage you will want to purchase one or more books that you can refer to at leisure. Some ideas are given in the books section of this site. Not all the books are currently in print, although as an addicted browser of second-hand bookshops, I have found it surprisingly easy to pick up good quality books relating to meteorology in this way. It is also worth asking about availability of titles from a good bookseller. In addition, the Internet now boasts several on-line book suppliers of both new and second-hand items.

If you are involved in sports / activities such as gliding, sailing, ballooning etc., the associations or clubs that you will probably belong to may offer courses, ad-hoc instruction etc. Contact them for details. The Royal Yachting Association (RYA) in particular encourages its members to be aware of weather processes, availability and interpretation of forecasts etc: visit their web site at: http://www.rya.org.uk

Residential study courses (e.g. Field Study Courses, Met Office College courses), are available, which are an excellent way to get to grips with meteorology in a somewhat deeper way, as well as enjoying some congenial company and the benefit of an experienced instructor. Find out about these from 'activity' magazines, good travel agents, newspaper weekend supplements etc. For the Met Office College courses, visit their web site at: http://www.metoffice.gov.uk

The Met Office Education section is well worth a visit, not only for information on teaching / learning resources, but also for some current data which may be of interest .. use the main Met Office url above (and follow the link via 'Education')'), or find them direct at:- http://www.metoffice.gov.uk/education/index.html

The BBC Weather Centre (which is closely allied to the Met Office) also has a host of useful information - far too much to list here, but if you are starting out on your hobby / passion of meteorology, give this site a go:- http://www.bbc.co.uk/weather/

Subscribe to one of the magazines listed in the periodicals section: 'Weather' or 'The Journal of Meteorology' will provide much of interest. Don't be put off if you are 'new' to the subject; there is much to appreciate about the daily changes in the atmosphere which surround us for which you don't need a degree in Maths! These magazines will also publicise new books coming on to the market, and 'Weather' in particular often carries articles that deal with elementary meteorology.

And of course the Internet itself is increasingly a help with self-education. Many of us are trying to work up information pages on basic meteorology ... There are a few articles here on this site, and this FAQ and its Glossary attempt to cover some topics that frequently appear in the newsgroup. Use a search engine to do a bit of hunting: many North American sites carry some elementary instruction.

Now, moving on to study of the subject on a professional level, then you need to decide in which area your interests lie. My job was part of what loosely can be regarded as 'operational meteorology', i.e. forecast (& allied advisory) services for the general public, aviation, maritime and commercial customers. But this is one small area of what we might regard as the atmospheric sciences discipline. The 'flavour of the moment' is of course the study of climatology, both in historical terms, ( trying to reconstruct the climate of centuries past to detect long-term trends ), and for the future - for example predicting how atmospheric gas composition will change due to industrial and other processes, and how these changes will affect the weather (and therefore us) in the decades and centuries to come. Atmospheric chemistry (for example the study of ozone in the high atmosphere) has an important part to play in the protection of human (and other) life on the planet from harmful solar radiation, as well as being important in understanding the heat budget of the atmosphere. Another specialism is the study of 'micro-climates' around mature woodlands, or in urban situations for example. Studies also increasingly cross formerly rigid disciplinary boundaries, such as into the realms of oceanography and vulcanology. And, under-pinning all, the modern 'weather' business would be nowhere without IT specialists - larger employers such as the Met Office, will give you basic training in meteorology, even though your work is more to do with complex coding of the web site.

Which line you intend to pursue will dictate the requirements in terms of preliminary study, basic qualifications etc. If you intend to go for the 'theoretical' meteorology side of things, with a view, for example to modelling the atmosphere in terms of the basic equations governing physics, then a solid grounding in mathematics and science is required, almost certainly to a degree standard. However, when dealing with practical (or applied) meteorology, then this standard need not be so rigorous. There are many avenues open now which approach the subject via the geography / environmental sciences route. A solid understanding of mathematics (and some physics) is desirable, but this can be 'bolted-on' at a later stage, rather than being a pre-requisite.

The field of opportunity is now so vast (and also changing quickly) that it would be best to investigate the requirements at an early stage: the Internet is an excellent source of such information. In particular, The Royal Meteorological Society see here) have some excellent advice on their web site ... to go directly to this page, go to http://www.royal-met-soc.org.uk/education.html

To find the latest information about University courses within the UK, and a host of other information about tertiary level education in the atmospheric sciences field, visit the Universities and Colleges Admissions Service (UCAS) site at http://www.ucas.co.uk/

Finally, a word of warning. If (as I did) you decide to go into 'front-line' forecasting, be prepared for some sleepless nights! Not only is shift-work required (& weekend working), but you must brace yourself to expect the disappointment of being woken at 4am to the sound of rain lashing against your window, when you confidently expected it to hold off until lunchtime at least: a thick skin, and a sense of humour are a requirement!

Maps, diagrams, leaflets, books, posters, videos etc., etc.... how can I get hold of these?

For books, it is always a good idea to give the reference section of your local lending library a chance. What you seek may be held on the shelves and you can search for titles, subjects etc., at many of the larger libraries and order them if they are not immediately available. Books long out of print, and not held in a public library may be found sometimes in second-hand book shops. The National Meteorological Library may also hold the item and for serious research, a call to them might be beneficial (see here).

Between them, the UK Met Office & Royal Meteorological Society offer a selection of charts, leaflets, educational packs and the like for sale which often will be of great help, for example to students and teachers in the subject. Other publications may also be obtained (if currently in print) from the Stationery Office (formerly HMSO).

  • The Met Office: on-line at http://www.metoffice.gov.uk/ and follow the link via "Education and Weather", or "Enquiries and comments". This latter link will allow you to email a request to the Information Officer. Alternatively, for callers within the United Kingdom telephone 0870 900 0100 (national rate call) to speak to the information officer (normal working hours only). Outside of the UK, use +44(0)1392 885680: appropriate international rates apply.
  • The Royal Meteorological Society: on-line at http://www.royal-met-soc.org.uk/ and follow the link to Publications. Full details of items, price, contact addresses etc., are to be found on this comprehensive site.
  • The Stationery Office: on-line at http://www.the-stationery-office.co.uk, and use the search facilities, typing in key-words as required. Books published by the former HMSO might still be available via this site.

 

What is the 'Shapiro-Keyser' cyclone model?

During cases of rapid cyclogenesis (see the Glossary), the long accepted 'Norwegian' frontal/cyclone development model is not appropriate. M.A. Shapiro and D. Keyser, in a paper published in 1990, proposed an alternative which has gained widespread acceptance. I am grateful to Dr. David Schultz (NSSL) for permission to quote the following from an article written (with co-author H. Wernli) for the 'Mariners Weather Log', which to my mind is an excellent summary of the differences between the 'classical' frontal depression model and that proposed for rapid cyclogenesis events ...

"The Norwegian cyclone model, so named to honor the Norwegian meteorologists (e.g. Bjerknes, Bergeron and Solberg) who first conceptualised the typical life-cycle of midlatitude cyclones in the 1910's and 1920's, presents the evolution of a cyclone from an incipient frontal wave with cold and warm fronts, to a deepening cyclone with a narrowing warm sector as the cold front rotates around the cyclone faster than the warm front, and finally to a mature cyclone with an occluded front. Typically, a Norwegian cyclone is oblong, orientated roughly north-south with the cold front more intense and longer than the weak and "stubby" warm front.

The Shapiro-Keyser cyclone model is named after the authors of the study that first presented this conceptual model of the frontal structure in some marine cyclones. As with the Norwegian cyclone model, an incipient cyclone develops cold and warm fronts, but in this case, the cold front moves roughly perpendicular to the warm front such that the fronts never meet, the so-called 'T-bone'. Also, a weakness appears along the poleward portion of the cold front near the low center, the so-called 'frontal fracture' and a back-bent front forms behind the low center. (In the final stage), colder air encircles warmer air near the low center, forming a warm seclusion. Typically, the Shapiro-Keyser cyclone is oblong, elongated east-west along the strong warm front".

Schultz & Wernli then go on to state (I paraphrase) ... an important factor in determining which evolution will be preferred ... is the nature of the large-scale (i.e. mid/upper tropospheric) flow. NWP experiments have indicated significant sensitivity to the profile of the wind speed across the jet flow and other studies have indicated that the along-jet variations of wind speed can be important. Cyclones embedded within diffluent flow (e.g. jet-exit regions) tend to evolve like the Norwegian cyclone model, whereas cyclones embedded within confluent flow (e.g. jet-entrance regions) tend to evolve like the Shapiro-Keyser cyclone model.

(for background to jets, entrance and exit regions etc., see here and various entries in this FAQ and its associated Glossary.)

"Thunder heard!": rumbles, crackles & bangs - why the difference?

The noise we hear when a thunderstorm is in progress is caused by the near-instantaneous expansion of air (due to intense heating) along the path of the lightning discharge. The type of noise we hear (rumble, crackle or bang) depends upon the distance of the observer from the lightning path and the path 'structure', e.g. simple stroke, branched pattern etc.

A lengthy drawn-out rumble/crackle of thunder is due to the slow speed of sound (330 m/s) through air - the lightning 'flash' of course travels at the speed of light (several orders of magnitude faster) - effectively our eyes see the discharge as soon as it happens; the sound wave takes much longer to reach our ears, unless the lightning strike is very close by - when we would experience a violent 'bang' . . and hopefully nothing worse!

When lighting undergoes much 'branching', both within and outside the cloud, although the whole flash would take place over a small fraction of a second, the sound waves from each part of the path (and the branches), take different periods of time to reach an observer: the sound will be heard as either a continuous crackling (relatively close-by storm), or low-intensity 'rumble' (distant storm), or a mix of the two, again depending upon the character of the discharge path.

The reason why near thunder 'cracks' and distant thunder 'rumbles' is this: sharp cracks are composed of sound waves with high frequency (or short wavelengths); these are rapidly damped owing to irregularities in wind & temperature. The lower frequency (longer wavelength) portion of the sound wave is not absorbed anything like as much, and therefore it is these we hear from distant storms.

Thunder, under normal atmospheric conditions, has an absolute maximum 'travel' of about 20 km, and more usually 8 to 10km. In mountainous areas and under conditions of 'anomalous' low-level refraction, then greater distances may be achieved. (See also here)

How are the seasons defined?

In the NE Atlantic / NW European area (and like locations), we are used to the idea of the four seasons: spring, summer, autumn and winter. For climatological 'accounting' purposes, these are defined using three calendar month blocks thus:-

  • March - May = spring
  • June - August  =  summer
  • September - November  =  autumn
  • December - February  =  winter.


However, terminology used in forecasts when describing temperature levels (in the UK) relative to average values (e.g. 'mild', 'rather cold', 'hot' etc.) employ a modified form of the above: the spring descriptions apply from mid-March to mid-May; summer-time is mid-May to mid-September; autumn from mid-September to mid-November and winter runs from mid-November to mid-March. Note though that as with the climatological seasons (above), in any one year, the 'bounds' may not be appropriate: they are only a guide and the terminology they allow do not find favour with all!

Moving away from the four 'classic' seasons, other periods have been suggested which fit more closely the various climatological phases of a year: for example, Hubert Lamb (1950), a noted British climatologist, proposed the following:

  • High summer: 18th June to 9th September
  • Autumn: 10th September to 19th November
  • Early winter: 20th November to 19th January
  • Late winter: 20th January to 29th March
  • Spring: 30th March to 17th June

This classification was based on his (and others, e.g. Brooks) analysis of British Isles weather periodicities.

And of course we must appreciate that our desire to neatly parcel the year into equal-length units (in the case of the 'standard' definition above), does not sit well when transferred to other parts of the world. For example, tropical (& adjacent) areas subject to the migration of the ITCZ will have a 'dry' season and a 'wet' season (perhaps two) - not necessarily of equal length. In south Asia, it is more sensible to talk about the periods affected by the various monsoon wind regimes, with short transition periods. In the interior of large continents, and at high (arctic / sub-arctic) latitudes, the "seasons" are observed only by convention: the year often consists of extended deep winter & high summer spells, again with short transitional periods of about a month.

One classification that does NOT find favour with meteorologists though is that used by astronomers. They use the dates of equinoxes and solstices to define the start/end of seasons.